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Vortex phase diagram of Bi2Sr2CaCu2O8+y probed by critical Josephson current
Physica C 378–381 (2002) 523–526 www.elsevier.com/locate/physc
Vortex phase diagram of Bi2Sr2CaCu2O8þy probed by critical Josephson current S. Ooi *, T. Mochiku, K. Hirata Nanomaterial Laboratory, National Institute for Materials Science, Sengen 1-2-1, Tsukuba 305-0047, Japan Received 27 September 2001; accepted 19 January 2002
Abstract To probe the vortex phase in Bi2 Sr2 CaCu2 O8þy single crystals in the field parallel to the c-axis, we have investigated the I–V characteristics of intrinsic Josephson junctions. Since the critical current Ic of junctions reflects the correlation of pancake vortices between the layers, the c-axis correlation of vortices could be probed by Ic measurements. In-line symmetric junctions fabricated by focused ion beam have been used to measure the I–V characteristics. An anomaly of Ic at the vortex lattice melting transition is actually observed at high temperatures. Additionally, other anomalies are also found by the field cooling measurements. The phase diagram obtained by Ic measurements is presented. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Ec; 74.72.Hs; 74.25.Dw; 74.25.Ha Keywords: Intrinsic Josephson junction; Vortex phase diagram; Bi2 Sr2 CaCu2 O8þy
1. Introduction In highly anisotropic high-Tc superconductors such as Bi2 Sr2 CaCu2 O8þy (BSCCO), so-called intrinsic Josephson junctions (IJJs) have attracted much attention because nanometer-scale junctions are naturally formed in crystal structures [1] without artificial fabrications of S–I–S junctions. Various studies have been performed concerning the properties of IJJs. One of the interesting issues is the investigation of the relation between the I–V characteristics of IJJs and the vortex phase in the
*
Corresponding author. Tel.: +81-298-59-2362; fax: +81298-59-2301. E-mail address: [email protected] (S. Ooi).
mixed state [2–4]. Since the critical current Ic of IJJs reflects the correlation of pancake vortices between the layers, the c-axis correlation of vortices could be probed by Ic measurements. For instance, a jump in the field dependence of Ic is expected in the field (Hm ), where the vortex lattice melting transition occurs since the measurements of the Josephson plasma resonance (JPR), which is an alternative method to observe the interlayer correlation of pancake vortices, have revealed the existence of the jump of Ic in Hm [5,6]. Actually, we have first succeeded to observe the anomaly at melting transition by I–V measurements in both mesa and in-line symmetric type junctions [7,8]. These results suggest that the Ic measurement of IJJs is a useful method to probe the vortex phase diagram.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 8 8 - 0
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S. Ooi et al. / Physica C 378–381 (2002) 523–526
A considerable issue for the vortex phase diagram in the field parallel to the c-axis is whether the vortex lattice melting transition occurs through two stages or not. Recently, several evidences have been reported from both experiments and simulations [9–12]. Fuchs et al. have first observed a new anomaly, which is called as Tx -line, in liquid regime in BSCCO [9]. Subsequently, it has been reported that the data from muon spin rotation is consistent with a two-stage melting scenario where the intraplanar melting of the vortex lattice and the interplanar decoupling of the vortex lines occur separately [10]. Additionally, the vortex slush phase has been found between the vortex glass and vortex liquid phases in the case of the anisotropy c ¼ 20 by the Monte Carlo simulations [12]. Their results indicate that the vortex slush–vortex lattice transition is of first order and there is a finite jump of the phase difference between the nearestneighbor superconducting planes at the transition [12]. Although it is expected that a jump of the phase difference might be observed at the phase boundary, any jumps were not observed by the JPR measurements [5,6]. Since the JPR measurements is limited in underdoped samples, in which plasma frequency is low enough to be measured, Ic measurement is another helpful method to investigate the phase diagram in various doping levels. To study the vortex phase diagram in the field parallel to the c-axis we have measured I–V characteristics of IJJs in BSCCO. The vortex phases are discussed from results of the field and temperature dependence of Ic .
2. Experimental Single crystals of BSCCO were grown by traveling-solvent floating-zone technique [13]. A platelet of single crystals was cut into narrow strips by a dicing machine. The width and length of the strips are 50–55 lm and 2–3 mm, respectively. Silver pastes were put on the surface and the samples were annealed at 500 °C in the air for 10 min to form electrodes. The contact resistance of the electrodes was several X. After that, the center of strips of the single crystals was cut by focused
Fig. 1. A resistance curve as a function of temperature of the in-line symmetric junction. The area of the measured junction is 7:3 13 lm2 with a thickness of about 2 lm. The inset shows a schematic picture of a sample. Black part in the center of the picture indicates IJJs.
ion beam to fabricate the in-line symmetric IJJs with superconducting electrodes. The shape of the junction is schematically shown in the inset of Fig. 1. I–V measurements have been performed by current-driven mode with a four-probe configuration. In measurements, magnetic fields are always applied along the c-axis.
3. Results and discussion Temperature dependence of the resistance in the junction is shown in Fig. 1. The resistance after the fabrication of the junction with 7:3 13 lm2 is 1.3 kX at room temperature while the resistance was smaller than 1 X before the fabrication. Therefore, the measured resistance is considered as the c-axis resistance of the junction part. The caxis resistivity is evaluated as 6.2 X cm. The onset of the superconducting transition temperature is 86 K. This junction is overdoped because of the annealing process. Fig. 2 shows the I–V characteristics of the junction at the fields up to 435 Oe by 5 Oe step. The magnetic field was swept from minus to plus
S. Ooi et al. / Physica C 378–381 (2002) 523–526
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Fig. 2. I–V characteristics of the junction in the fields from 0 to 435 Oe by 5 Oe step at 70.0 K. Each curve is shifted to right by a 0.1 lV step for clarity. The bold solid line indicates the I–V curve near the boundary between the vortex lattice and liquid states.
fields. Ic is determined as the current value by the voltage criterion of 0.1 mV. Around the zero field Ic takes maximum, corresponding curve is shown at the left side in the figure. As the field increases, Ic gradually decreases. In an intermediate field we can observe a clear change of I–V characteristics. The boundary is shown by the bold line in the figure. Ic is suddenly enhanced below this field, where the vortex lattice melting transition occurs. In the liquid phase the current cannot flow without dissipation and finite voltages appear even at small current regime. Ic extracted from I–V curves is plotted as a function of magnetic field in Fig. 3. Except the data at 80 K clear kink structures are observed at intermediate fields Hm . Hm gradually increases with decreasing temperature, saturates at 40 to 20 K, and decreases at lower temperatures than 20 K. Since the temperature dependence and the value of Hm are similar to that of the well-known first order melting transition and the peak effect above 20 K, the kink corresponds to the transition. In this junction the sharp enhancement of Ic is observed even at 5 K.
Fig. 3. Field dependence of Ic at 5 K (top) and the other temperatures (bottom). Curves are properly shifted along the vertical axis to clarify. The doted lines connect the points (Hm ) where kink structures are observed. Ic is enhanced in the inside region surrounded by the doted lines, which corresponds to the vortex lattice phase.
To recheck this phase boundary Hm , temperature dependence of Ic was measured in the fieldcooling process. The data in various fields are shown in Fig. 4. Clear enhancements of Ic (Tm ), which can be considered as the vortex lattice freezing transition, are observed in the low fields of 100–750 Oe around 0.2–0.3 mA. Even above 800 Oe a small enhancement of Ic is still observed as followed by Tm line. Another hump structure (Td ) appears around 20 K. Below 15 K, Ic rapidly reduces in all magnetic fields. The field dependences of both Tm and Td are plotted in Fig. 5 with the temperature dependence of Hm . Concerning the strange decrease of Hm at low temperatures obtained from the measurements by the fieldsweep process, we could not observe the anomaly corresponding to Hm by the field-cool measurements.
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S. Ooi et al. / Physica C 378–381 (2002) 523–526
The data of Tm in Fig. 5 are not consistent with those of Hm above 800 Oe and its behavior is similar to that of Tx in Ref. [9]. Therefore, it is possible that the enhancement of Ic at Tm in higher fields occurs due to the vortex slush–vortex liquid transition as suggested by the simulations [12]. However, since there remains an ambiguity of the voltage criterion used to determine Ic , further examinations will be needed to confirm whether Tm indicates a phase boundary. On the other hand, another boundary Td may relate to the depinning transition because it has the topological similarity in the phase diagram [9]. Our results suggest that the interlayer correlation of vortices becomes large below Td because Ic once increases below this temperature.
Fig. 4. Temperature dependence of Ic in various fields by fieldcooling measurements. The solid line marked as Tm shows the starting points of the enhancements of Ic . The slight increases of Ic are still observed even in the high fields, which are indicated by the doted line. The solid line marked as Td connects the points of another increases, which are observed at low temperatures near 25 K.
4. Conclusion To study the vortex phase diagram in BSCCO single crystals the critical Josephson current Ic has been measured in wide range of magnetic field and temperature by using the in-line symmetric IJJs. We could observe the anomaly corresponding to the first order melting transition. Additionally, other anomalies, which may be considered as a transition in liquid state (Tx ) and the depinning transition, were found. At these boundaries lower temperature phases have larger interlayer phase coherence.
References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 5. The vortex phase diagram obtained by the Ic measurements of IJJs. The solid circles (Hm ) show the field where the kink structures are observed in field dependence of Ic . The white triangles and squares show Tm and Td which are defined in Fig. 4.
[9] [10] [11] [12] [13]
R. Kleiner et al., Phys. Rev. Lett. 68 (1992) 2394. Sh. Luo et al., Phys. Rev. B 51 (1995) 6655. M. Suzuki et al., Phys. Rev. Lett. 81 (1998) 4248. A. Yurgens et al., Phys. Rev. B 59 (1999) 7196. T. Shibauchi et al., Phys. Rev. Lett. 83 (1999) 1010. M.B. Gaifullin et al., Phys. Rev. Lett. 84 (2000) 2945. K. Hirata, S. Ooi, T. Mochiku, Physica C 335 (2000) 139. S. Ooi, T. Mochiku, K. Hirata, Physica C 362 (2001) 269. D.T. Fuchs et al., Phys. Rev. Lett. 80 (1998) 4971. T. Blasius et al., Phys. Rev. Lett. 82 (1999) 4926. R. Sugano et al., Physica C 357–360 (2001) 428. Y. Nonomura, X. Hu, Phys. Rev. Lett. 86 (2001) 5140. T. Mochiku et al., Physica C 282–287 (1997) 475.
Vortex phase diagram of Bi2Sr2CaCu2O8þy probed by critical Josephson current S. Ooi *, T. Mochiku, K. Hirata Nanomaterial Laboratory, National Institute for Materials Science, Sengen 1-2-1, Tsukuba 305-0047, Japan Received 27 September 2001; accepted 19 January 2002
Abstract To probe the vortex phase in Bi2 Sr2 CaCu2 O8þy single crystals in the field parallel to the c-axis, we have investigated the I–V characteristics of intrinsic Josephson junctions. Since the critical current Ic of junctions reflects the correlation of pancake vortices between the layers, the c-axis correlation of vortices could be probed by Ic measurements. In-line symmetric junctions fabricated by focused ion beam have been used to measure the I–V characteristics. An anomaly of Ic at the vortex lattice melting transition is actually observed at high temperatures. Additionally, other anomalies are also found by the field cooling measurements. The phase diagram obtained by Ic measurements is presented. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Ec; 74.72.Hs; 74.25.Dw; 74.25.Ha Keywords: Intrinsic Josephson junction; Vortex phase diagram; Bi2 Sr2 CaCu2 O8þy
1. Introduction In highly anisotropic high-Tc superconductors such as Bi2 Sr2 CaCu2 O8þy (BSCCO), so-called intrinsic Josephson junctions (IJJs) have attracted much attention because nanometer-scale junctions are naturally formed in crystal structures [1] without artificial fabrications of S–I–S junctions. Various studies have been performed concerning the properties of IJJs. One of the interesting issues is the investigation of the relation between the I–V characteristics of IJJs and the vortex phase in the
*
Corresponding author. Tel.: +81-298-59-2362; fax: +81298-59-2301. E-mail address: [email protected] (S. Ooi).
mixed state [2–4]. Since the critical current Ic of IJJs reflects the correlation of pancake vortices between the layers, the c-axis correlation of vortices could be probed by Ic measurements. For instance, a jump in the field dependence of Ic is expected in the field (Hm ), where the vortex lattice melting transition occurs since the measurements of the Josephson plasma resonance (JPR), which is an alternative method to observe the interlayer correlation of pancake vortices, have revealed the existence of the jump of Ic in Hm [5,6]. Actually, we have first succeeded to observe the anomaly at melting transition by I–V measurements in both mesa and in-line symmetric type junctions [7,8]. These results suggest that the Ic measurement of IJJs is a useful method to probe the vortex phase diagram.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 8 8 - 0
524
S. Ooi et al. / Physica C 378–381 (2002) 523–526
A considerable issue for the vortex phase diagram in the field parallel to the c-axis is whether the vortex lattice melting transition occurs through two stages or not. Recently, several evidences have been reported from both experiments and simulations [9–12]. Fuchs et al. have first observed a new anomaly, which is called as Tx -line, in liquid regime in BSCCO [9]. Subsequently, it has been reported that the data from muon spin rotation is consistent with a two-stage melting scenario where the intraplanar melting of the vortex lattice and the interplanar decoupling of the vortex lines occur separately [10]. Additionally, the vortex slush phase has been found between the vortex glass and vortex liquid phases in the case of the anisotropy c ¼ 20 by the Monte Carlo simulations [12]. Their results indicate that the vortex slush–vortex lattice transition is of first order and there is a finite jump of the phase difference between the nearestneighbor superconducting planes at the transition [12]. Although it is expected that a jump of the phase difference might be observed at the phase boundary, any jumps were not observed by the JPR measurements [5,6]. Since the JPR measurements is limited in underdoped samples, in which plasma frequency is low enough to be measured, Ic measurement is another helpful method to investigate the phase diagram in various doping levels. To study the vortex phase diagram in the field parallel to the c-axis we have measured I–V characteristics of IJJs in BSCCO. The vortex phases are discussed from results of the field and temperature dependence of Ic .
2. Experimental Single crystals of BSCCO were grown by traveling-solvent floating-zone technique [13]. A platelet of single crystals was cut into narrow strips by a dicing machine. The width and length of the strips are 50–55 lm and 2–3 mm, respectively. Silver pastes were put on the surface and the samples were annealed at 500 °C in the air for 10 min to form electrodes. The contact resistance of the electrodes was several X. After that, the center of strips of the single crystals was cut by focused
Fig. 1. A resistance curve as a function of temperature of the in-line symmetric junction. The area of the measured junction is 7:3 13 lm2 with a thickness of about 2 lm. The inset shows a schematic picture of a sample. Black part in the center of the picture indicates IJJs.
ion beam to fabricate the in-line symmetric IJJs with superconducting electrodes. The shape of the junction is schematically shown in the inset of Fig. 1. I–V measurements have been performed by current-driven mode with a four-probe configuration. In measurements, magnetic fields are always applied along the c-axis.
3. Results and discussion Temperature dependence of the resistance in the junction is shown in Fig. 1. The resistance after the fabrication of the junction with 7:3 13 lm2 is 1.3 kX at room temperature while the resistance was smaller than 1 X before the fabrication. Therefore, the measured resistance is considered as the c-axis resistance of the junction part. The caxis resistivity is evaluated as 6.2 X cm. The onset of the superconducting transition temperature is 86 K. This junction is overdoped because of the annealing process. Fig. 2 shows the I–V characteristics of the junction at the fields up to 435 Oe by 5 Oe step. The magnetic field was swept from minus to plus
S. Ooi et al. / Physica C 378–381 (2002) 523–526
525
Fig. 2. I–V characteristics of the junction in the fields from 0 to 435 Oe by 5 Oe step at 70.0 K. Each curve is shifted to right by a 0.1 lV step for clarity. The bold solid line indicates the I–V curve near the boundary between the vortex lattice and liquid states.
fields. Ic is determined as the current value by the voltage criterion of 0.1 mV. Around the zero field Ic takes maximum, corresponding curve is shown at the left side in the figure. As the field increases, Ic gradually decreases. In an intermediate field we can observe a clear change of I–V characteristics. The boundary is shown by the bold line in the figure. Ic is suddenly enhanced below this field, where the vortex lattice melting transition occurs. In the liquid phase the current cannot flow without dissipation and finite voltages appear even at small current regime. Ic extracted from I–V curves is plotted as a function of magnetic field in Fig. 3. Except the data at 80 K clear kink structures are observed at intermediate fields Hm . Hm gradually increases with decreasing temperature, saturates at 40 to 20 K, and decreases at lower temperatures than 20 K. Since the temperature dependence and the value of Hm are similar to that of the well-known first order melting transition and the peak effect above 20 K, the kink corresponds to the transition. In this junction the sharp enhancement of Ic is observed even at 5 K.
Fig. 3. Field dependence of Ic at 5 K (top) and the other temperatures (bottom). Curves are properly shifted along the vertical axis to clarify. The doted lines connect the points (Hm ) where kink structures are observed. Ic is enhanced in the inside region surrounded by the doted lines, which corresponds to the vortex lattice phase.
To recheck this phase boundary Hm , temperature dependence of Ic was measured in the fieldcooling process. The data in various fields are shown in Fig. 4. Clear enhancements of Ic (Tm ), which can be considered as the vortex lattice freezing transition, are observed in the low fields of 100–750 Oe around 0.2–0.3 mA. Even above 800 Oe a small enhancement of Ic is still observed as followed by Tm line. Another hump structure (Td ) appears around 20 K. Below 15 K, Ic rapidly reduces in all magnetic fields. The field dependences of both Tm and Td are plotted in Fig. 5 with the temperature dependence of Hm . Concerning the strange decrease of Hm at low temperatures obtained from the measurements by the fieldsweep process, we could not observe the anomaly corresponding to Hm by the field-cool measurements.
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S. Ooi et al. / Physica C 378–381 (2002) 523–526
The data of Tm in Fig. 5 are not consistent with those of Hm above 800 Oe and its behavior is similar to that of Tx in Ref. [9]. Therefore, it is possible that the enhancement of Ic at Tm in higher fields occurs due to the vortex slush–vortex liquid transition as suggested by the simulations [12]. However, since there remains an ambiguity of the voltage criterion used to determine Ic , further examinations will be needed to confirm whether Tm indicates a phase boundary. On the other hand, another boundary Td may relate to the depinning transition because it has the topological similarity in the phase diagram [9]. Our results suggest that the interlayer correlation of vortices becomes large below Td because Ic once increases below this temperature.
Fig. 4. Temperature dependence of Ic in various fields by fieldcooling measurements. The solid line marked as Tm shows the starting points of the enhancements of Ic . The slight increases of Ic are still observed even in the high fields, which are indicated by the doted line. The solid line marked as Td connects the points of another increases, which are observed at low temperatures near 25 K.
4. Conclusion To study the vortex phase diagram in BSCCO single crystals the critical Josephson current Ic has been measured in wide range of magnetic field and temperature by using the in-line symmetric IJJs. We could observe the anomaly corresponding to the first order melting transition. Additionally, other anomalies, which may be considered as a transition in liquid state (Tx ) and the depinning transition, were found. At these boundaries lower temperature phases have larger interlayer phase coherence.
References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 5. The vortex phase diagram obtained by the Ic measurements of IJJs. The solid circles (Hm ) show the field where the kink structures are observed in field dependence of Ic . The white triangles and squares show Tm and Td which are defined in Fig. 4.
[9] [10] [11] [12] [13]
R. Kleiner et al., Phys. Rev. Lett. 68 (1992) 2394. Sh. Luo et al., Phys. Rev. B 51 (1995) 6655. M. Suzuki et al., Phys. Rev. Lett. 81 (1998) 4248. A. Yurgens et al., Phys. Rev. B 59 (1999) 7196. T. Shibauchi et al., Phys. Rev. Lett. 83 (1999) 1010. M.B. Gaifullin et al., Phys. Rev. Lett. 84 (2000) 2945. K. Hirata, S. Ooi, T. Mochiku, Physica C 335 (2000) 139. S. Ooi, T. Mochiku, K. Hirata, Physica C 362 (2001) 269. D.T. Fuchs et al., Phys. Rev. Lett. 80 (1998) 4971. T. Blasius et al., Phys. Rev. Lett. 82 (1999) 4926. R. Sugano et al., Physica C 357–360 (2001) 428. Y. Nonomura, X. Hu, Phys. Rev. Lett. 86 (2001) 5140. T. Mochiku et al., Physica C 282–287 (1997) 475.